In a typical petrochemical facility having a steam cracker for the pyrolysis of ethane, propane, naphtha, gas oil and other suitable steam cracking feedstock, the effluent from the steam cracker contains acid gases, such as carbon dioxide, hydrogen sulfide and traces of carbonyl sulfide, in addition to desirable olefin products such as ethylene and propylene. In order to recover the desired products, it is necessary to purify the steam cracker effluent to remove the acid gases, for example, by contacting the effluent in an absorption tower with a suitable solvent, such as an aqueous alkanolamine solution. In the absorption tower, the acid gases are absorbed by the aqueous alkanolamine solution to produce an alkanolamine rich solution, which is withdrawn from the absorption tower. The alkanolamine rich solution is then sent to a regenerator where the alkanolamine rich solution is heated to drive off most of the acid gases. The alkanolamine lean solution exiting the regenerator is recycled back to the absorption tower to be contacted with additional steam cracker effluent. Meanwhile, the acid gases may be further upgraded in a sulfur recovery unit to obtain a sulfur product that may be sold.
The problems of polymer formation in acid gas systems, i.e., formation of acid gas byproducts and fouling of units, are well known in the prior art. For example, U.S. Pat. No. 3,696,162 describes polymerization problems encountered in the alkanolamine regenerator and heat exchange system that are caused by the presence of dienes in steam cracker effluent entering the absorption tower and complexes formed from the dienes and acid gas anions. This reference addresses the polymerization problem by introducing a hydrocarbon solvent with the aqueous alkanolamine solution into the absorption tower to absorb the dienes and to act as a solvent for any complexes thereof. In this process, an alkanolamine rich solution phase and a hydrocarbon solvent phase containing absorbed dienes and complexes are formed in the bottom of the absorption tower upon settling. The reference indicates that the alkanolamine rich solution phase and the hydrocarbon solvent phase containing absorbed dienes and complexes may be withdrawn separately or together. If withdrawn together, a portion of the hydrocarbon solvent phase would eventually be separated to provide a surge stream to prevent a buildup of dienes and complexes in the system. The alkanolamine rich solution phase and hydrocarbon solvent phase are then passed together through a heat exchanger and regenerator. The hydrocarbon solvent is described as generally having an initial boiling point of about 80° C., with an aromatic solvent being preferred.
In the system described in U.S. Pat. No. 3,696,162, the alkanolamine solution and the hydrocarbon solvent are circulated together through the absorption tower, heat exchanger and regenerator. Since the hydrocarbon solvent of U.S. Pat. No. 3,696,162 generally has an initial boiling point of about 80° C., contamination of the regenerator overhead occurs and the quality of the sulfur recovered from the acid gases in the sulfur recovery unit is poor. Although U.S. Pat. No. 3,696,162 addresses the polymerization problems when removing acid gases from steam cracker effluent, this reference fails to recognize the problem of contamination of the regenerator overhead with the hydrocarbon solvent and the associated deterioration of the sulfur product that is recovered in the sulfur recovery unit when such contamination occurs.
Typically, acid gases in refinery gas are removed as part of a refinery operation prior to being upgraded in a sulfur recovery unit. Often times, it may be desirable to utilize the adsorption tower used to remove acid gases from the steam cracker effluent for the additional service of absorbing acid gases from the refinery gas. This may be accomplished, for example, by combining the steam cracker effluent with the refinery gas, prior to contact with an aqueous alkanolamine solution in the absorption tower. The acid gases recovered from the steam cracker effluent and the refinery gas may then be upgraded in the sulfur recovery unit.
Applicants have found that when the absorption tower is used for the additional service of absorbing acid gases from refinery gas, the polymerization problems are of a different nature than those described in U.S. Pat. No. 3,696,162, such that the solvent described in U.S. Pat. No. 3,696,162 as generally having an initial boiling point of about 80° C. does not effectively reduce fouling of the regenerator and heat exchangers. Specifically, the polymers that form when absorbing acid gases from steam cracker effluent and refinery gas are of a higher molecular weight and range from a thick syrup-like liquid to rock-hard solid. Such polymers may be a combination of amine degradation products, carbonyl polymers, diene polymers, free radical polymers and corrosion products, and may range in molecular weight from 150 to 10,000.
Accordingly, there is a desire to reduce fouling of the regenerator and heat exchangers when the absorption tower is used to remove acid gases from a mixed feed of steam cracker effluent and refinery gas, while avoiding contamination of the regenerator overhead with the hydrocarbon solvent in order to improve the quality of sulfur recovered in the sulfur recovery unit. Further, there is a desire to easily separate the hydrocarbon solvent phase from the alkanolamine rich solution.